TEM specimens usually have thicknesses of a few nanometers so as to be transparent to electrons. Different preparation techniques can be used depending on the material to be examined and the problems to be solved. Conventional methods hereby include applying a nanoscale suspension on special TEM grids as well as powder preparation, wherein the sample to be tested is present as a finely atomized powder material. Another possibility for preparing extremely thin layers is based on cleavable or cuttable sample materials. Thin layers of a crystalline material along the lattice planes of a material can be produced with the so-called small-angle cleavage technique (SACT), whereas for example with ultra-microtomy amorphous biological samples embedded in a casting resin can initially be brought into a solid, cuttable form and subsequently surface-machined in an ultra-microtome.
The aforementioned methods are supplemented by various methods for selective thinning of individual material regions of the samples. This thinning can be done in various ways, wherein electrolytic thinning is widely used for metallic materials. Chemical etching or processing with a focused ion beam (FIB) is also common. For many materials, purely mechanical methods, for example methods using fine grinding and fine polishing discs, optionally with the aid of a polishing agent, can also be used.
Over the past years, many of these methods have been constantly improved and optimized. In addition to commercially available special equipment for thinning, tools for manual mechanical fine polishing processing are also known. However, all prior art methods use ion thinning for final fine polishing of the produced thin-film sample surfaces, with the exception of ultra-microtomy, cleaving and electrolytic thinning. However, this partly causes considerable amorphization of existing crystalline regions. These amorphous layers produce, inter alia, severe noise in the TEM images. Depending on the material, the electron beam may also cause recrystallization which makes it considerably more difficult to interpret the examined object. It is therefore the aim of any type of sample preparation to prevent the formation of such amorphous layers. Furthermore, the additional ion bombardment and etching introduces unwanted artifacts which may make it much more difficult to interpret the TEM results.
Although a sample surface prepared by ion thinning has extremely small surface roughness, this process should not be used because of these problems. Although the aforementioned cleaving and cutting process can make the additional use of ion thinning (or FIB in particular) largely unnecessary, it cannot be used with all types of samples and have severe limitations for spatially resolved investigations. Accordingly, various mechanical abrasion processes are particularly advantageous for the vast majority of samples.
One of the most common polishing methods is mechanical thinning of samples by using a machine having a rotating grinding wheel placed on the sample. In order to produce an optimal homogenous material removal, the sample is additionally continuously rotated under the abrasion spot, so that the thinned region forms a concave trough profile. The size of the produced troughs depends mainly on the size and thickness of the grinding wheel and its position relative to the sample surface.
However, when using such trough grinders, the sample spots thinned with such a trough grinder have disadvantageously an inhomogeneous thickness, so that an ideal image can be formed with TEM only within a very small area around the trough center or the inner trough edge. Such a process is very time consuming, in particular for spatially resolved studies on specific regions of material samples due to the considerable effort associated with the preparation. Moreover, it often impossible to deliberately prepare the desired locations especially of inhomogeneous samples.
The invention is therefore based on the object to provide a sample carrier for thinning by way of machine-based mechanical fine polishing, which obviates or at least significantly ameliorates one or more of the aforedescribed problems of the prior art in the preparation of suitable samples for transmission electron microscopy (TEM). In particular, a method for thinning specimens for TEM is to be provided, which generally does not require the use of FIB or ion thinning and which also allows, as a result of large-scale removal of material, a spatially resolved examination of inhomogeneous samples following only a single preparation step.